Charged particle beam columns are typically employed in scanning electron microscopy (SEM), which is a known technique widely used in the manufacture of semiconductor wafers, being utilized in a CD metrology tool, the so-called CD-SEM (critical dimension scanning electron microscope), and a defect review SEM (DF-SEM). In SEM, the region of a sample to be examined is two-dimensionally scanned by means of a focused primary beam of electrically charged particles, usually electrons. Irradiation of the sample with the primary electron beam releases secondary (and/or backscattered) electrons. The secondary electrons are released at that side of the sample at which the primary electron beam is incident, and move back to be captured by a detector, which generates an output electric signal proportional to the so-detected electric current. The energy and/or the energy distribution of the secondary electrons is indicative of the nature and composition of the sample.
Various prior art CD-SEMs and method for measuring critical dimensions are illustrated in the following U.S. patent applications which are incorporated herein by reference: U.S. patent application publication number 20030015699 of Su, titled “Integrated critical dimension control for semiconductor device manufacturing”; U.S. patent application publication number 20050048654 of Wu, titled “Method of evaluating reticle pattern overlay registration”; U.S. patent application publication number 20040173746 of Petrov, et al., titled “Method and system for use in the monitoring of samples with a charged particles beam”; U.S. patent application publication number 20040056207 of Petrov, et al., titled “Deflection method and system for use in a charged particle beam column”; U.S. patent application publication number 20030218133 of Petrov, et al., titled “Charged particle beam column and method for directing a charged particle beam”, and U.S. patent application publication number 20030209667 of Petrov, et al., titled “Charged particle beam apparatus and method for inspecting samples.
SEM includes such main constructional parts as an electron beam source (formed with a small tip called “electron gun”), an electron beam column, and a detector unit. The detector unit may be located outside the path of the primary beam propagation through the column, or may be located in the path of the primary beam (the so-called “in-column” or “in-lens” detector). The electron beam column includes, inter alia, a beam focusing/deflecting arrangement formed by a lens assembly and a deflector assembly. The deflection of the primary beam provides for scanning the beam within a scan area on the sample, and also for adjusting incidence of the primary beam onto the sample (an angle of incidence and/or beam shift), as well as directing the secondary beam to the detector.
In SEM, in order to reduce the “spot” size of the electron beam (up to nanometers) and thus increase the image resolution, a highly accelerated electron beam is typically produced using accelerating voltages of several tens of kilovolts and more. Specifically, the electron optic elements are more effective (i.e. produce smaller aberrations) when the electrons are accelerated to high kinetic energy. However, in order to avoid damaging a sample (resist structure and integrated circuit) that might be caused by such a highly energized electron beam, the electron beam is decelerated just prior to impinging onto the sample. Deceleration of the electrons can generally be accomplished by selectively creating a potential difference between the pole piece of a magnetic objective lens and the sample. Alternatively, the same effect can be achieved by actually introducing electrodes having selective potential applied thereto.
Some systems of the kind specified utilize the lens assembly in the form of a combination of a magnetic objective lens and an electrostatic lens, the so-called “compound magnetic-electrostatic lens” (e.g., EP 1238405 and EP 1045425, both assigned to the assignee of the present application). The electrostatic part of the compound magnetic-electrostatic lens is an electrostatic retarding lens (with respect to the primary charged particle beam), and has electrodes held at different potentials, one of the two electrodes being formed by an anode (which is typically in the form of a tube defining a primary beam drift space for the primary beam propagation to the sample, such as anode 11 of FIG. 2-7) arranged within a magnetic objective lens along its optical axis, and the other electrode being a metallic cap provided below the magnetic objective lens. The sample actually presents the third electrode of the electrostatic lens. The electric field created by the electrostatic lens in the vicinity of the sample appropriately decelerates the primary beam and also facilitates the extraction of secondary charged particles from the sample. U.S. Pat. No. 5,780,859 of Feuerbaym et al., which is incorporated herein by reference, describes a prior art electrostatic-magnetic lens arrangement.
Various optical characteristics of the SEM relate to the working distance between the SEM and the sample. The working distance should be large enough to guarantee that the wafer and the SEM do not tough each other during the scanning of the wafer but on the other hand larger working distances result in lower resolution as well as increased aberrations.
In addition, larger working distances require larger holes/apertures, thus relatively large amount of electrostatic field leaks through the aperture and can reduce the SEM resolution and accuracy.
There is a need to provide an efficient charged particle beam system and a method for inspecting wafers.